I was using naturally generated bedrock to contain a wither at the roof of the nether. The wither was stuck inside and surrounded horizontally by a 3 x 3 square of bedrock. It was standing on top of a cobblestone wall. I approached the area in 1.11.1, and as the chunks loaded and I got close enough, noticed that the wither was taking a lot of damage, which it wasn't supposed to be. I'm not sure if it was suffocating or if the damage was self-inflicted. It seemed to be firing off a lot more wither skulls than normal, which is another possible source of the damage if they were striking the bedrock in the wither's immediate location. I decided to re-log to see if it was just a bug. I re-loaded the world in 1.10.2. This time, the wither broke the blocks in its surroundings (I didn't see if it broke the bedrock, though I suspect it didn't) and escaped.

My best guess for the cause of the problem is that when I loaded the chunks, the wither loaded in the wrong location - inside the cobblestone wall instead of on top of it. This could have led to the apparent suffocation damage and the subsequent destruction.